High-Speed Machining

Pending EPA regulations on metal cutting fluids have made dry machining a hot topic recently. And while the need for dry machining may be apparent, issues including the perceived inability to cut dry at higher speeds and the changeover costs involved result in dry machining being perceived as impractical by most manufacturers.

This is not the case-high-speed dry machining in milling applications on both cast iron and aluminum is possible. By examining the roughing, holemaking and finishing processes involved with the dry machining of a cast iron engine head, it becomes apparent that the key is a balance between advanced metal cutting strategies, special tooling and the machine tool specifications. Integrated correctly manufacturers can realize the accuracy and productivity of high-speed machining, along with the benefits of dry machining.

The challenge of heat dissipation without coolant requires a completely different approach to tooling. Special tooling utilizing high-performance coatings, heat-resistant materials and through-spindle air are required. Since milling cuts are necessarily interrupted, forming discontinuous chips, we can take advantage of the cooling cycle while the cutting surface is not engaged with the material. This reduces the temperature of the insert or allows the surface footage to be increased at a given temperature threshold. And it also benefits the, workpiece. Especially in applications using aluminum, dry milling addresses the melting and smearing that can occur when the aluminum workpiece absorbs excessive heat.
The key to dry machining an example part, the cast iron cylinder head for a 4.3 liter engine, is the use of two advanced metal cutting techniques Red Crescent and Flush Fine machining. Red Crescent machining resulted from Makino's realization that a combination of high feedrate and high spindle rpm reduces, rather than increases, the thrust forces against the workpiece. For example, this application uses HSK-63A with high pressure, through-spindle air running up to 14,000 rpm. With feed rates of up to 1,575 ipm the thrust forces are reduced by as much as 75 to 90 percent in some instances.

In dry machining, ceramic, silicone nitride or CBN cutting materials are required to combat the intense heat. Due to fabrication issues with these cutting materials, these materials are only available in insert form which limits the application to inserted mills, drills and boring bars. Red Crescent techniques concentrate intense heat immediately in front of the tool. This elevates metal temperature in the affected area of the block to 600-700¡C and creates a visible red arc that gives the technique its name. And while this intense heat eliminates Red Crescent milling from use on aluminum workpieces, the metal removal rates achieved in cast iron approach rates typically associated with aluminum processing.

This intense heat allows for high-efficiency machining. The heat plasticizes the cast iron, greatly reducing its yield strength and increasing metal removal efficiency by a factor of 3 to 1 when compared to conventional roughing operations. Typically this much heat would distort the part, but because Red Crescent feedrates are so high most of the heat is retained in the chip and removed before the heat can soak into the workpiece. This makes the workpiece more thermally stable and, as a result, more dimensionally accurate.

Heat dissipation during holemaking is addressed by using special coated carbide tools in a process called Flush Fine machining. Flush Fine is a high-speed, high-definition and low heat milling process patented by Makino. It combines high spindle speeds, a high feedrate with a shallow depth of cut to mill holes rather than drilling them.

Flush Fine is ideal for most shallow holes with up to a 5 to 1 length to diameter (l to d) ratio. For the bulk of holemaking in automotive applications, this technique can be used. Holes with a depth greater than a 5 to 11 to d ratio require more conventional tooling. Drill geometries are then used with special coatings to improve temperature resistance and lubricity. For these tools it is also required to employ pseudo-dry techniques, using minute amounts of coolant that evaporate during the application and leave no residue.

Flush Fine uses precisely controlled high-pressure, through-spindle air for chip disposal and to prevent heat buildup in the workpiece or the tool and prevent the re-cutting of chips. To achieve the most aggressive metal removal in this process, the through-spindle air requires a higher level of pressure than is available at most shops. Boosting shop air pressure as high as 160 to 200 PSI provides superior cooling and chip disposal. High-feed machining provides a high enough feed rate, relative to the specific heat and conductivity of the material, to reduce the temperature rise of the workpiece by approximately 50%. This results in less thermal expansion in the workpiece.

Using Makino's patent-pending Typhoon or Tornado tools, holes of various diameters and depths can be milled, chamfered, counter-bored and threaded with a single tool. This reduces non-cut time associated with numerous tool changes. This process uses high-speed, high accuracy helical interpolation of a small diameter tool to shape and thread the holes. By interpolating a small diameter end mill, holes can be formed to different diameters and depths with one tool.
The Typhoon or Tornado tool's accuracy relies on the machining center's Geometric Intelligence software, improved algorithms to predict and compensate for machine dynamics, helical interpolation and sophisticated control hardware. This includes highspeed CNC with improved block processing time.

Low thrust machining strategies are applied to finish operations to minimize static deformation of the workpiece, fixturing or machine tool. These strategies rely on low density cutters with fewer inserts and lower feed per revolution and high surface footage. This strategy also reduces the fixturing pressure for simpler fixturing which provide access to more facets of prismatic parts by eliminating buttresses, work supports and clamps. The capital cost of the machining center can also be reduced as there are fewer fixtures necessary for dry applications.

In face milling of the engine head, low density, full-top PCBN inserts with sintered joints are used for more aggressive metal removal. The PCBN inserts allow for a faster cutting speed with higher operating temperature, while a conventional PCBN insert with a brazed joint limits productivity due to the potential melting of the brazed joint from the excessive heat in this application. And this metal removal is achieved using lower thrust levels and high feedrates which result in less defamation of the work piece and higher accuracy. Cleaning of the workpiece is also easier as it isn't contaminated by fluids.

In this application, switching from carbide to PCBN allows 100% spindle load cuts with only two inserts. Fewer inserts combined with longer tool life translate into fewer tool setups, while the higher metal removal rates allow for shorter cycle time and increased productivity. PCBN inserts cost more than ceramic and silicone nitrate inserts and much more than carbide inserts being used at conventional surface footage. However, longer tool life and a reduced insert count can result in a lower expendable tooling cost per piece.

While tooling challenges have been the immediate focus of dry machining, it is a balance between tooling, the machining center and the cutting strategies that make it a reality. The machine tool in particular has been overlooked as an element requiring special preparation.

Traditionally, machining dry at low-speeds is the norm, so when moving to high-speeds the machining center becomes an even larger part of the focus. Spindle horsepower, servo power and chip disposal technology must be examined when switching to dry. This application requires a high-speed, highly rigid spindle that can handle high rpm and high horsepower to achieve the fast metal removal rates. Eliminating coolant will also prove beneficial to some horizontal machining center spindles, increasing their performance.

The machining center's augers and chip conveyors must be checked to make sure they can effectively flush dry chips out of the machine without fluid. In some cases, augers may need to be added or chip conveyors enhanced. Special telescoping covers and seals are also required to protect the machining center's ballscrews and ways. If these critical components are not protected, dry chips can damage them. And, since the chips aren't contaminated by coolant, they can be more easily recycled and are more valuable.

Approached incorrectly, changing over from wet to dry operations can be costly and problematic. Optimum implementation requires machine tools designed for dry machining with proper options, guarding and air handling systems. This approach, over time, is more flexible than switching over existing machining centers, making dry machining feasible.

Changing over from wet to dry operations positively impacts the capital cost of the machining center. By replacing a mist collector with a dust collector, replacing coolant pumps with additional air compressor capacity (and a lubricant metering unit for pseudo dry applications), the capital cost can be less than when purchasing a high-speed machining center for wet operations. Additional costs are also driven out at the expense level. Coolant management and disposal costs are eliminated, cutting tool savings can be realized and even the cost of electricity for running the air compressor can be less than the cost of running the coolant pump.

Implemented correctly, dry machining is not cost prohibitive. With the anticipated EPA regulations, some shops are already pursuing dry machining to get involved with the implementation and overall learning curve. Needless to say, the technology for dry machining at high speeds does exist. The key is to keep in mind it is a combination of cutting tool, machining center configuration and machining strategies are required for a successful installation. A thorough understanding of all three in regards to the application are necessary to leverage positive results and cost savings. Based on your specific application, it may be time you investigated dry machining as well.